Atomic-Scale Observations of (010) LiFePO₄ Surfaces Before and After Chemical Delithiation

The ability to view directly the surface structures of battery materials with atomic resolution promises to dramatically improve our understanding of lithium (de)­intercalation and related processes. Here we report the use of state-of-the-art scanning transmission electron microscopy techniques to probe the (010) surface of commercially important material LiFePO₄ and compare the results with theoretical models. The surface structure is noticeably different depending on whether Li ions are present in the topmost surface layer or not. Li ions are also found to migrate back to surface regions from within the crystal relatively quickly after partial delithiation, demonstrating the facile nature of Li transport in the [010] direction. The results are consistent with phase transformation models involving metastable phase formation and relaxation, providing atomic-level insights into these fundamental processes

Structural Understanding of Superior Battery Properties of Partially Ni-Doped Li₂MnO₃ as Cathode Material

We examined the crystal structures of Li₂(Ni_<i>x</i>Mn_1–<i>x</i>)­O_3(−δ) (<i>x</i> = 0, 1/10, 1/6, and 1/4) to elucidate the relationship between the structure and electrochemical performance of the compounds using neutron and synchrotron X-ray powder diffraction analyses in combination. Our examination revealed that these crystals contain a large number of stacking faults and exhibit significant cation mixing in the transition-metal layers; the cation mixing becomes significant with an increase in the Ni concentration. Charge–discharge measurements showed that the replacement of Mn with Ni lowers the potential of the charge plateau and leads to higher charge–discharge capacities. From a topological point of view with regard to the atomic arrangement in the crystals, it is concluded that substituting Mn in Li₂MnO₃ with Ni promotes the formation of smooth Li percolation paths, thus increasing the number of active Li ions and improving the charge–discharge capacity

Dependence of Structural Defects in Li₂MnO₃ on Synthesis Temperature

Li₂MnO₃, an electrode material for Li ion batteries, belongs to the <i>C</i>2/<i>m</i> space group and is known to have a cubic-close-packed (<i>ABC</i>...) layered structure, in which the transition-metal layer is supposed to have an ordered atomic arrangement with Li atoms at the 2<i>b</i> site and Mn atoms at the 4<i>g</i> site. However, recently, it has been reported that this compound usually does not exhibit such an ideal structure and instead contains a large number of structural defects, not only stacking faults but also mixing of Li and Mn atoms between the 2<i>b</i> and 4<i>g</i> sites. To elucidate the effect of such structural defects on the electrochemical behavior, we examined the crystal structure of Li₂MnO₃ synthesized at various temperatures by simultaneously analyzing the stacking faults and cation mixing using FAULTS, a Rietveld code. Our examination showed that the crystals consist of both disordered and ordered domains; the disordered domains contain a large number of stacking faults along the <i>c</i> axis and have considerable Li/Mn atomic mixing within the transition-metal layer, whereas the ordered domains have almost no defects. At low synthesis temperatures, the disordered domains are dominant. However, the ordered domains increase at the expense of the disordered domains above 770 °C and become dominant at higher temperatures. It is also found that the degree of cation mixing in the disordered domains remains almost constant irrespective of synthesis temperature. The crystalline defects such as stacking faults or Li/Mn cation mixing are expected to promote the formation of smooth Li percolation paths. The decreasing of the disordered domains leads to dramatically decreased capacity. This indicates that the observed capacities of Li₂MnO₃ can be determined by the relative amounts of the ordered/disordered domains in the structure

Hierarchically Structured Thermoelectric Materials in Quaternary System Cu–Zn–Sn–S Featuring a Mosaic-type Nanostructure

Multinary chalcogenide semiconductors in the Cu–Zn–Sn–S system have numerous potential applications in the fields of energy production, photocatalysis and nonlinear optics, but characterization and control of their microstructures remains a challenge because of the complexity resulting from the many mutually soluble metallic elements. Here, using state-of-the-art scanning transmission electron microscopy, energy dispersive spectroscopy, first-principles calculations and classical molecular dynamics simulations, we characterize the structures of promising thermoelectric materials Cu₂(Zn,Sn)­S₃ at different length scales to gain a better understanding of how the various components influence the thermoelectric behavior. We report the discovery of a mosaic-type domain nanostructure in the matrix grains comprising well-defined cation-disordered domains (the “tesserae”) coherently bonded to a surrounding network phase with semiordered cations. The network phase is found to have composition Cu_4+<i>x</i>Zn_<i>x</i>Sn₂S₇, a previously unknown phase in the Cu–Zn–Sn–S system, while the tesserae have compositions closer to that of the nominal composition. This nanostructure represents a new kind of phonon-glass electron-crystal, the cation-disordered tesserae and the abrupt domain walls damping the thermal conductivity while the cation-(semi)­ordered network phase supports a high electronic conductivity. Optimization of the hierarchical architecture of these materials represents a new strategy for designing environmentally benign, low-cost thermoelectrics with high figures of merit